Spray coating enhances 3D processes

More and more applications require lithographic patterns in, on, over or near severe topographical features >10 microns and often up to 300 microns in vertical height or depth from the wafer surface. Current spin coating approaches create unusable films with striations, streaks, pinholes and other anomalies. Dry laminated films have been used but are limited in the achievable resolution to the range of their thickness. Electroplated resists have also been considered, but are costly to deposit and limited to conductive surfaces. Finally, spray coating, a technology well known in automotive industry, has been proven to be the solution for this challenge. This article explores a novel spray coating technology and its capability to deposit highresolution resist films on different 3D microstructures.

Resist film deposition across severe topography
Spin coating is an established method for resist deposition on planar surfaces or over a surface, which has a slight unevenness common in most processes in CMOS technology. For MEMS applications 3D structures are required for the functionality of the system (Fig 1). The structures, which consist of trenches, V-grooves or holes, are the cause for defects in the resist layer using ordinary spin coating technology. This means behind a structure, an area of nonuniform resist is generated by centrifugal force. In the worst case areas are not even covered by resist. This process-dependent limitation of spin coating for generating a homogeneous resist layer on top of a non-planarized surface explains the need of an alternative technology. Electro deposition (ED) of photo resist provides a solution for the problem. The main advantage of ED is the coating of a conformal resist layer independent of the geometry on the wafer surface. This technology has however also some important disadvantages. The process is selflimiting so that a given resist thickness can be obtained irrespective of the topography. Furthermore the bath solutions have a short lifetime, which generates high material consumption and raises process costs and this method works only on conductive surfaces.

Behavior of liquids onnon-planar surfaces
The main problem has been that the resist tends to pull back from sharp topography edges. Since the resist is not dry, when it is deposited on the surface, it shows the same behavior like a liquid. Two forces are responsible for the effect (Fig. 2.). On the one hand the gravity reduces the film thickness on the edge, since the resist tends to flow down the slope. On the other hand the surface tension of the resist pulls the resist back from the corner and leads to a bead formation (Fig. 2. upper left). This process depends on parameters like viscosity of the resist or contact angle between surface and resist [2].

In a corner we see the opposite effect. The liquid resist flows down the slope and material fills the corner. The formation of resist in the corner is defined by the surface tension of the used material, where the resist tries to form an ideal surface, which is as small as possible.

Spray coating - a technology to enhance 3D processes
A modified spray coating technology has been developed by SUSS MicroTec with which the described problems have been successfully reduced (Fig. 2). As a consequence there is also hardly any resist accumulation in trenches or V-grooves (Fig. 3). There is a trade-off between resist surface smoothness and flow characteristics. Best results across severe topography are obtained with low resist flow after deposition. Tab. 1 shows an overview of materials, which have been already sprayed. Beside the shown structures coating a sharp 90° edge is the biggest challenge - even for spray coating. The resist tends to flow down the stiff step and breaks the resist film on the edge. Fig. 4. demonstrates the good coverage of a trench with a 90° sidewall using a positive resist AZ4620 from AZ Electronic Materials to protect the structure in the following etch processes (Fig. 4.). Micro lenses are the basic element of building up devices for MOEMS applications.

The dry etch production process of micro lenses generates a non-planar surface, which for certain applications has the need of a conformal deposition of resist. A detail of a single lens is shown (Fig. 5. large image) as well as the overall coating quality over the lenses (Fig. 5. lower right).

Patterning of spray coated resist
For patterning resist across deep steps a lithography system with a very large depth of focus capability is required. The normal diffraction optic in the SUSS Mask Aligner [3] can be used up to a gap of 70µm. For patt rning a 150µm (Fig. 6.) and a 300 µm (Fig. 7.) KOHetched cavity, special "LEGO" optics (LargeExposure Gap Optic) is used. The deeper the step is the more difficult it is to guarantee same line width on the top and on the bottom of the structure. A continuity in line width has been demonstrated over a 150 µm step. Some packaging technologies involving image sensors as well as some advanced IC chip designs require wiring steps that run up hills or through deep trenches, with height changes in the order of up to 300 µm.

Conclusion
The performance of this novel spray coating technology has been demonstrated by application results, which show a conformal resist deposition over 90° corners, KOH-etched cavities, Vgrooves, trenches and micro lenses. The results achieved by a SUSS AltaSpray coating system (Fig. 8.) prove that the flow characteristics of deposited photo resist has been reduced and that spray coating allows the full coverage of sharp edges in severe topography applications.

Resist is not pulling back from such edges even in cases of edge angles up to 90°. In addition, the accumulation of resist in trenches that plagued both conventional spin coating as well as commercially available spray coating does not occur with this method.